The present invention relates to a process for producing Phillips catalysts in which an oxidic support material is treated in suspension with a chromium salt solution and subsequently, after removing the solvent, calcined in an oxygen-containing atmosphere at above 300xc2x0 C.
A process of this type is known and is comprehensively described in, for example, DE-A 25 40 279. The catalysts produced as described there are also comminuted and have particle sizes in the range from 20 to 2,000 xcexcm, in particular from 40 to 300 xcexcm.
DE-A 36 40 802 and DE-A 36 40 803 state that restricting the oxidic support to a particular, very narrow particle size distribution in the range from 50 to 150 xcexcm gives chromium trioxide catalysts which give polymers having an improved particle morphology at equal or higher catalyst productivity.
Finally, it has been found in U.S. Pat. No. 5,641,842 that Phillips catalysts having particle sizes of  greater than 75 xcexcm are advantageous for the morphology of polyethylene prepared therewith.
After evaluation of the relevant literature, it can be said in summary that classification of the oxidic support material influences the catalyst productivity and the polyethylene morphology. According to the literature, the best results are generally achieved using relatively coarse catalysts, i.e. those having particle sizes of  greater than 50 xcexcm.
It is an object of the present invention to provide a new process which makes it possible to produce Phillips catalysts which further increase the productivity of the polymerization of ethylene in loop precipitation processes and, in particular, allow increased polyethylene solids contents within the polymerization reactor.
We have found that this object is achieved by a process of the generic type mentioned at the outset, whose defining features are that the oxidic support material before calcination and/or the catalyst after calcination are/is comminuted until a mean particle size of  less than 100 xcexcm has been reached and the proportion of particles having a size of  less than 50 xcexcm is at least 30%.
According to the present invention, the oxidic support material used is a silica gel which has a solids content, calculated as silicon dioxide, in the range from 10 to 30% by weight, preferably from 11 to 25% by weight, and is largely spherical. Such a silica gel is obtained by introducing a solution comprising sodium water glass or potassium water glass into a twisting stream of a mineral acid longitudinally and tangentially to the flow direction of the stream and spraying the silicic acid hydrosol formed into a gaseous medium so as to form droplets. The sprayed hydrosol then solidifies in the gaseous medium to form spherical particles and is freed of adhering salts by washing with water.
The spherical hydrosol is then treated with an organic liquid selected from among alcohols having from 1 to 4 carbon atoms until at least 60% of the water present in the hydrosol has been extracted. The dewatered hydrogel which has been treated with the alcoholic liquid is then dried until at  greater than 160xc2x0 C. using an inert carrier gas the residual alcohol content is less than 10% by weight.
The xerogel obtained in this way is then loaded with chromium from a 0.05-5% strength by weight solution of chromium trioxide in a ketone having from 3 to 5 carbon atoms or from a 0.05-15% strength by weight solution of a chromium compound which is converted into chromium trioxide under the conditions of the calcination in an alcohol having from 1 to 4 carbon atoms and the solvent is subsequently evaporated under reduced pressure.
For the calcination, the chromium-laden oxidic support material is maintained at from 300 to 1,100xc2x0 C. in a water-free, oxygen-containing gas stream for from 10 to 1,000 minutes.
The comminution according to the present invention of the oxidic support material or of the catalyst material obtained as described above is carried out by dry milling using a ball mill or in a beater mill as described, for example, in DE-A 36 40 802. The milling time necessary to achieve the desired particle size is determined by taking samples at particular time intervals.
In the olefin polymerization in which the catalyst produced according to the present invention is used, it is possible to prepare homopolymers of ethylene or copolymers of ethylene with a comonomer having from 3 to 12 carbon atoms in an amount of up to 10% by weight of comonomer. The polymerization itself is carried out at from 30 to 150xc2x0 C. under a pressure in the range from 0.2 to 15 MPa.
It has surprisingly been found that, at a constant reactor output, the Phillips catalysts having a particle size of  less than 100 xcexcm used according to the present invention result in an increase in the average residence time of the catalyst in the reactor and that the catalyst productivity increases at the same time. The higher catalyst productivity presumably results from significantly higher polyethylene solids concentrations being able to be achieved in the loop precipitation process, particularly in loop reactors, when using the catalysts produced according to the present invention than when using the catalysts described in the literature, which customarily have particle sizes of  greater than 100 xcexcm. Furthermore, it has surprisingly been found that the Phillips catalysts produced according to the present invention make it possible to achieve comparable results in terms of the morphology of the polyethylene prepared therewith to those obtained using the conventional catalysts having particle sizes of  greater than 100 xcexcm.
The process of the present invention gives particularly optimal results when the proportion of particles having a size of  less than 50 xcexcm is in the range from 40 to 80%.
For the purposes of the present invention, all particle size data were determined in accordance with DIN 53 477, sieve analysis.